Deprecation Notice: This post doesn't work as of ansible 2. The API was changed.
This covered in the Ansible documentation under "Python API."
For example, ansible -i hosts dbservers -m setup is implemented via:
import ansible.runner
runner = ansible.runner.Runner(
   module_name='setup',
   module_args='',
   pattern='dbservers',
)
dbservers_get_facts = runner.run()
There are a bunch of non-documented parameters in the __init__ method of Runner (from ansible.runner).  There's too many to list inline, but I've included some of the parameters in this post as a guess to what you're specifically looking for.
class Runner(object):
    ''' core API interface to ansible '''
    # see bin/ansible for how this is used...
    def __init__(self,
        host_list=C.DEFAULT_HOST_LIST,      # ex: /etc/ansible/hosts, legacy usage
        module_path=None,                   # ex: /usr/share/ansible
        module_name=C.DEFAULT_MODULE_NAME,  # ex: copy
        module_args=C.DEFAULT_MODULE_ARGS,  # ex: "src=/tmp/a dest=/tmp/b"
        ...
        pattern=C.DEFAULT_PATTERN,          # which hosts?  ex: 'all', 'acme.example.org'
        remote_user=C.DEFAULT_REMOTE_USER,  # ex: 'username'
        remote_pass=C.DEFAULT_REMOTE_PASS,  # ex: 'password123' or None if using key
        remote_port=None,                   # if SSH on different ports
        private_key_file=C.DEFAULT_PRIVATE_KEY_FILE, # if not using keys/passwords
        sudo_pass=C.DEFAULT_SUDO_PASS,      # ex: 'password123' or None
        ...
        sudo=False,                         # whether to run sudo or not
        sudo_user=C.DEFAULT_SUDO_USER,      # ex: 'root'
        module_vars=None,                   # a playbooks internals thing
        play_vars=None,                     #
        play_file_vars=None,                #
        role_vars=None,                     #
        role_params=None,                   #
        default_vars=None,                  #
        extra_vars=None,                    # extra vars specified with he playbook(s)
        is_playbook=False,                  # running from playbook or not?
        inventory=None,                     # reference to Inventory object
        ...
        su=False,                           # Are we running our command via su?
        su_user=None,                       # User to su to when running command, ex: 'root'
        su_pass=C.DEFAULT_SU_PASS,
        vault_pass=None,
        ...
        ):
For instance, the above command that specifies a sudo user and pass would be:
runner = ansible.runner.Runner(
   module_name='setup',
   module_args='',
   pattern='dbservers',
   remote_user='some_user'
   remote_pass='some_pass_or_python_expression_that_returns_a_string'
)
For playbooks, look into playbook.PlayBook, which takes a similar set of initializers:
class PlayBook(object):
    '''
    runs an ansible playbook, given as a datastructure or YAML filename.
    ...
    '''
    # *****************************************************
    def __init__(self,
        playbook         = None,
        host_list        = C.DEFAULT_HOST_LIST,
        module_path      = None,
        .... 
and can be executed with the .run() method. e.g.:
from ansible.playbook import PlayBook
pb = PlayBook(playbook='/path/to/book.yml, --other initializers--)
pb.run()
more robust usage can be found in the ansible-playbook file.
As far as I know, translating playbooks to Python modules is a bit more involved, but the documentation listed above should get you covered and you can reuse the YAML parser built into Ansible to convert playbooks to variables.